Engineers Mine Rare Earth Elements from Fertilizer Byproduct

Engineers at Pennsylvania State University (Penn State) have received a National Science Foundation grant to recover rare earth elements (REEs) from phosphogypsum, a fertilizer byproduct stored indefinitely in open dumps and pumped into designated lakes.

Rare earth elements are ubiquitous and are an important component of microchips, for example. Their rarity is due to the fact that they are more difficult to extract. Traditional means of obtaining them usually produce large carbon emissions and a large portion of REEs are lost in the waste of industrial processes.

To develop a more sustainable process for extracting REEs from phosphogypsum, a fertilizer byproduct, Penn State researchers have been awarded a four-year, $571,658 National Science Foundation grant for a total of $1.7 million as part of a collaboration with Case Western Reserve University and Clemson University.

Each university has separate funding for specific aspects of the project, but the project is being coordinated centrally by Case Western Reserve researchers. Lauren Greenlee, associate professor of chemical engineering, and Rui Shi, assistant professor of chemical engineering, are co-leading the effort at Penn State.

"Today, in Florida alone, an estimated 200,000 tons of rare earth elements are trapped in untreated phosphogypsum waste," Greenlee said, explaining that phosphogypsum is piped into ditches and ponds for indefinite storage. This source of REEs is currently untapped because of the challenges associated with radioactive species and the difficulty of separating individual elements.

The vision of this project is to discover new separation mechanisms, materials and processes to recover valuable resources, including REEs, fertilizers and clean water, from waste streams of the fertilizer industry, paving the way for a sustainable supply of REEs and a sustainable agricultural sector in the country".

Greenlee also noted that the U.S. relies heavily on international sources for rare earth element supplies and that the COVID-19 outbreak has caused lengthy delays in the supply chain.

Greenlee said, "This is a significant issue, complicated by the economic, environmental and safety complexities of obtaining and using rare earth elements internationally."

Formed when phosphate rock is processed into fertilizer, phosphogypsum contains small amounts of naturally occurring radioactive elements, such as uranium and thorium. Because of this radioactivity, this byproduct is stored indefinitely, and improper storage can contaminate soil, water and the atmosphere. To harvest the REEs in phosphogypsum, researchers proposed a multi-stage process using engineered peptides capable of precisely identifying and separating REEs through specialized membranes.

"Individual rare earth elements have similar dimensions and the same formal charge, so traditional membrane separation mechanisms are not sufficient," Greenlee said. "A key technical goal of this research is to discover the mechanisms that underpin peptide ion selectivity and to use these mechanisms to design a new class of highly selective membranes."

Once the peptides have been developed, Greenlee will study them in an aqueous solution, and Shi will use systems analysis tools, including techno-economic analysis and life cycle assessment, to evaluate the environmental impact and economic viability of the proposed rare earth element recovery system under a variety of design and operating conditions.

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